ECOLOGY AND TAXONOMY OF Halimeda 179 



What any system of classifying life-forms is bound to do is to recog- 

 nize common adaptations to common environmental constraints. But 

 these systems can easily mislead. Raunkiaer (1934), for instance, 

 remarks on an absence of phanerophytes (essentially trees) from the 

 tundra. This absence of trees in the Arctic may be accounted for by 

 arguments based on heat balance, desiccation or maintenance of bio- 

 mass, yet Raunkiaer's classification must have convinced many 

 students that the critical difficulty is exposing buds to cold air, a propo- 

 sition for which there is no evidence. In the same way there seems to be 

 no reason for suggesting that the environmental range of a species of 

 Halimeda is directly responsible for imposing a particular system of 

 perennation. The showing in at least two of Feldmann's perennating 

 taxa of a single species of Halimeda is evidence that this approach is of 

 limited use. 



3. Some conclusions about macroscopic growth 



Halimedae grow a segment at a time. A segment begins from the 

 medullary (nodal) filaments of the preceding segment which grow out 

 as whitish filaments. These filaments branch and rebranch forming first 

 a whitish cone, then a segment-shaped mass of branching bundles of 

 filaments. The tips of these outgrowing filaments become the peripheral 

 utricles and the external surface of the plant. A new segment may be 

 fairly completely formed in about a day. It turns noticeably green by 

 the end of the second day, and by this time has started to calcify. The 

 length and width of the segment are essentially fixed at this stage, but 

 the segment becomes increasingly calcified with age within certain limits, 

 and also may thicken somewhat. Several segments may be produced by 

 a frond in a week, although this varies with the species. Rates of growth 

 for some incrassata thalli of 6 1*4 mm wk"^ were obtained. 



The youngest segments are at the periphery of the plant, the oldest 

 at the base. Therefore, a gradient of segment ages exists in the alga. 



A Halimeda thallus may lose some fronds even as others are actively 

 growing. The moribund segments first lose their colour, turn white, may 

 attract epiphytes, then fall off, a process reminiscent of the shedding of 

 leaves by terrestrial plants. The resulting Halimeda fitter is responsible 

 for much of the mass of carbonate in coral reefs. 



Whole Halimeda plants may die, and the process is essentially the 

 same as the death of a frond. In addition, the holdfast becomes loose in 

 the sand, if the species is a Rhipsalian one, because rhizoidal filaments 

 have stopped growing. For species like incrassata, simulans and monile 

 there seems to be a definite pattern of youth, growth, old age and death. 



